1. Field of the Invention
The present invention relates to polyamide fluidized-bed coating powders, methods of making, and methods of use.
2. Discussion of the Background
Polyamide powders based on nylon-11 and nylon-12 and specifically developed for the fluidized-bed coating process generally have a median grain diameter d 50 to DIN EN ISO 4610 of from 95 to 120 μm. They generally have a bulk density to DIN 53 466 of from 400 to 600 g/l.
The powders are supplied as precipitated or ground powders made from the corresponding granular polyamide. Ground powders are produced by grinding in a mill, and precipitated powders are produced by dissolving the granular material in a solvent and precipitating, as in DE 29 06 647 B1. The latter process gives a polyamide powder with round grain shape, which is a consequence of the precipitation process. Unlike the precipitated powder, the ground powder has a sharp-edged grain.
Examples of precipitated nylon-12 powders are VESTOSINT® 1101, 1111, 1121, 1141, and 1161 (Degussa AG), and examples of ground nylon-11 powders for fluidized-bed coating are Rilsan® T 7260 gray and/7050 white (Atofina).
In the fluidized-bed coating process, hot metal parts are immersed into a pan with fluidizing polymer powder. The powder sinters on the hot metal surface and coalesces to give a homogeneous coating. A precondition for this is that the metal surface has a temperature above the melting point of the polymer powder.
Typically, a polyamide coating in a fluidized-bed coating process has a layer thickness of from 200 to 500 μm, or in specific instances even up to about 1000 μm. Layers thinner than about 200 μm are impossible or very difficult to produce by the fluidized-bed coating process using conventional fluidized-bed coating powders.
In coating technology, experience has led to acceptance that when using the polyamide fluidized-bed-coating powders commonly encountered in the market the thickness of a smooth coherent film has to be at least 2×d 50 of the powder. Since commercially available fluidized-bed-coating powders have a d 50 value of from 95 to 120 μm, the resultant lower limit for coating is about 200 μm.
The difficulties with lower film thicknesses are a consequence of immersion time and object temperature. On the one hand, the hot object can be immersed only briefly in order that there is no excessive growth of layer thickness on the surface, but on the other hand the powder needs a certain minimum immersion time in the bath if all of the corners and edges of the particular object are to be covered so that there are no uncoated defective areas.
For thin coatings of from 50 to 200 μm, therefore, use is frequently made of processes other than fluidized-bed coating, examples being ES spraying, high-temperature spraying, tribo spraying, and minicoating.
Minicoating is the most similar to fluidized-bed coating. Here, hot metal parts fall into a polyamide powder bed which, however, does not become fluidized. The metal parts pick up powder for as long as there is enough energy to melt the polymer. Initially a rough coating forms, since the component does not possess sufficient heat to make the slightly sintered powder coalesce to give a homogenous layer. If a smooth homogeneous surface is desired, this can be achieved by post-heating in an oven or irradiating with a heat source.
This process has been used especially for small and light metal parts, for example for corsetry clips. Examples of typical minicoating powders are VESTOSINT® 1164, 1174, and 2157 (Degussa) and Rilsan® 1452 MAC (Atofina). The median grain size d 50 of these powders is typically from 20 to 70 μm.
The bulk density of minicoating powders is mostly somewhat lower than for fluidized-bed coating powders. It is generally above 300-350 g/l.
Typical minicoating powders have markedly poorer fluidizing properties than specific fluidized-bed-coating powders, since their bulk density is lower and their grains are finer. Commercially available fine powders and minicoating powders have limited applicability in the typical fluidized-bed coating process.
However, there has recently been increased demand for polyamide powders intended to achieve thin layers in the range from 50 to 200 μm in the fluidized-bed coating process.
An example of an application of this type is the chromium-free coating of metal pipes for corrosion protection in the automotive industry, for example as described in U.S. Pat. No. 6,276,400 B1. For continuous pipe coating as in U.S. Pat. No. 6,276,400 B1 the traditional minicoating process is unsuitable. The process requires a fluidized-bed coating pan through which the pipe is continuously drawn. In this process there is local heating of the pipe to a temperature above the melting point of the polymer, preferably by induction.
The coating of pipes of this type for the automotive industry necessitates stringent requirements for layer thickness and coating homogeneity. On the one hand, the layer thickness should be as small as possible to save weight, but on the other hand the polyamide layer serves as corrosion protection on a component important for safety (brake piping, fuel piping) and therefore has to have absolutely no defects or variations in mechanical properties.